1. Technical Field
The techniques described herein relate to photovoltaic systems, power converters, associated circuits and related techniques for extracting energy from photovoltaic panels.
2. Discussion of the Related Art
There are several factors limiting the widespread adoption of solar power. The total installed cost ($/W) and total cost of ownership ($/Wh) have been well-studied as key metrics controlling grid penetration. Among the factors impacting installed cost per Watt are power converter cost and system efficiency, both of which share strong relations to converter and system complexity. A significant factor impacting the cost of ownership is the lifetime of the power converter and associated replacement costs. Cost-effective solutions for solar energy extraction should address system cost and complexity, conversion and tracking efficiencies and converter lifetimes.
The amount of power extracted from a photovoltaic (PV) module depends upon the operating point at which the PV module is operated. The amount of power extracted from the PV module is optimized when the product of the current and voltage at the output of the PV module is maximized. To extract the maximum amount of power from the PV panel, the current and voltage at the output of the PV panel can be set so that the PV panel operates at its maximum power point. However, the maximum power point can change over time, due to factors such as the amount of sunlight received and the age of the PV panel. A maximum power point tracking (MPPT) algorithm embedded in the control of a converter or an inverter can be used to control the power electronics so that the current and voltage at the PV module tracks the maximum power point.
In a grid-tied system, 120 Hz power ripple at the PV panel terminals negatively impacts the tracking ability of the MPPT algorithm. This problem may be addressed by adding a large electrolytic capacitor to filter out the ripple. However, the limited lifetime of electrolytic capacitors contends directly with the long-life characteristic desired for cost-effective solar conversion. To reconcile this, the “ripple port” inverter has been proposed, which still directly interfaces with the PV unit, but directs the 120 Hz ripple power to a transformer-coupled ripple port and away from the PV unit.